In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring to block fuel flow through the nozzle orifices. In many fuel systems, when the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle orifices, thus marking the beginning of injection. In another type of system, such as disclosed in U.S. patent application Ser. No. 686,491, now U.S. Pat. No. 5,676,114, filed Jul. 25, 1996, entitled Needle Controlled Fuel System With Cyclic Pressure Generation and commonly assigned to the assignee of the present invention, the beginning of injection is controlled by a servo-controlled needle valve element. The assembly includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve. U.S. Pat. No. 5,463,996 issued to Maley et al. discloses a similar servo-controlled needle valve injector.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. As a result, many proposals have been made to provide injection rate control devices in closed nozzle fuel injector systems. One method of controlling the initial rate of fuel injection is to spill a portion of the fuel to be injected during the injection event. For example, U.S. patent application Ser. No. 376,417, now U.S. Pat. No. 5,647,536, filed Jan. 23, 1995, entitled Injection Rate Shaping Nozzle Assembly for a Fuel Injector and commonly assigned to the assignee of the present application discloses a closed nozzle injector which includes a spill circuit formed in the needle valve element for spilling injection fuel during the initial portion of an injection event to decrease the quantity of fuel injected during this initial period thus controlling the rate of fuel injection. A subsequent unrestricted injection flow rate is achieved when the needle valve moves into a position blocking the spill flow causing a dramatic increase in the fuel pressure in the nozzle cavity. However, the needle valve is not servo-controlled and, thus, this nozzle assembly does not include a control volume for controlling the opening and closing of the needle valve. Moreover, the rate shaping nozzle assembly does not permit the rate to be selectively varied.
U.S. Pat. No. 5,133,645 to Crowley et al. discloses a common rail fuel injection system having two common rails serving respective banks of injectors. Fuel is supplied to each rail by a respective cam-operated reciprocating plunger pump. Each injector includes a nozzle element positioned in a spring cavity which receives high pressure fuel from the common rail via a check valve. The spring cavity is also connected, via an orifice, to a pressure control volume positioned above the nozzle element. A solenoid operated control valve opens to connect the control volume to drain thereby initiating injection as fuel flows from the nozzle cavity through the orifice to drain, and closes to terminate injection. U.S. Pat. No. 4,249,497 to Eheim et al. discloses a fuel injection system wherein fuel injection is controlled by controlling the differential pressure across a nozzle valve element using a single valve which opens to direct fuel to drain so as to start injection and closes to end injection. However, these references fail to disclose a means for achieving injection rate shaping.
U.S. Pat. No. 5,176,120 to Takahashi discloses a fuel injection system including a cam-operated fuel pump for supplying high pressure fuel to a common rail serving an injector. The injector includes a needle valve movable under the influence of differential fuel pressures as controlled by a solenoid-actuated valve. The system provides a control unit for achieving different fuel injection rates. However, the control unit must vary the pressure in the common rail to vary the fuel injection rate. When a lower common rail pressure is desired, the common rail fuel pressure is gradually lowered by the slow incremental extraction of fuel for injection without the addition of fuel to the rail. As a result, this system is incapable of quickly varying the pressure in the common rail to achieve a desired corresponding injection pressure and injection rate. Moreover, injection rate of each injector cannot be controlled independently. In addition, this system only permits two injection rate shapes thus limiting the effectiveness of the system. Also, the servo-controlled needle valve and actuator valve assembly is unnecessarily complex.
U.S. Pat. No. 2,959,360 to Nichols discloses a fuel injector nozzle assembly incorporating passages in the nozzle assembly for diverting the fuel from the nozzle assembly. Specifically, Nichols discloses a nozzle valve element having an axial passage formed therein for diverting fuel from the nozzle cavity into an expansible chamber formed in the nozzle valve element. A plunger is positioned in the chamber to form a differential surface creating a fuel pressure induced seating force on the nozzle valve element to aid in rapidly seating the valve element. The Nichols reference does not suggest the desirability of controlling the rate of injection.
Although some systems discussed hereinabove create different stages of injection, further improvement is desirable. None of the above discussed references disclose a fuel injector incorporating a simple, cost effective rate shaping device for a servo-controlled needle valve which minimizes the complexity of the nozzle assembly while effectively controlling emissions by controlling the rate of fuel injection.